Drempels
http://www.geisswerks.com/drempels/index.html
Drempels makes colorful, swirling, hallucinogenic patterns that resemble
a hurricane or tornado. It normally runs in your Windows background or
"desktop", replacing your old still-image wallpaper with gently-animating
visuals... and meanwhile, you can use your computer just like you normally
would. It uses very little CPU overhead - so little that it even runs
smoothly on a 233 MHz system!
Drempels can also be used as a screensaver; in this mode, it goes
fullscreen (like a normal screensaver) instead of rendering to the
desktop/background.
/CoMa